IB Physics Doppler effect Practice Questions

Question 1

As a sound source moves toward a stationary observer, the observed frequency is:

Accepted Answer

Higher than the emitted frequency. Wavefronts bunch up ahead of an approaching source, shortening the observed wavelength and raising the frequency.

Question 2

Light from a distant galaxy is observed to be redshifted. This indicates the galaxy is:

Accepted Answer

Receding from us. Redshift means the observed wavelength is longer, so the galaxy is receding (moving away from us).

Question 3

As a source of waves recedes from an observer, the observed wavelength is:

Accepted Answer

Longer than emitted. For a receding source the wavefronts spread out, so the observed wavelength increases.

Question 4

Which technology relies on the Doppler effect?

Accepted Answer

Speed-measuring (Doppler) radar. Doppler radar and Doppler ultrasound use frequency shifts to measure speed (of vehicles, blood flow, etc.).

Question 5

A car horn emits a steady note. As the car passes a stationary listener, the listener hears the pitch:

Accepted Answer

Drop from high to low as it passes. While approaching, the pitch is higher; as it passes and recedes, the pitch drops. So the pitch falls as the car goes by.

Question 6

The Doppler effect occurs because of relative motion between the:

Accepted Answer

Source and the observer. The Doppler shift depends on the relative motion between the source of the waves and the observer.

Question 7

As a wave source approaches an observer, the observed frequency _____.

Accepted Answer

increases / rises / goes up. Approaching sources give a higher observed frequency (blueshift for light, higher pitch for sound).

Question 8

The shift to longer wavelengths in light from a receding galaxy is called a _____shift.

Accepted Answer

red. A redshift indicates recession; the wavelength is stretched toward the red end of the spectrum.

Question 9

A sound source moves along the line joining observers X and Y, toward X. Which statements are correct? I More wavefronts pass X per second. II Wavefront speed is greater at X. III Wavelength is shorter at X.

Accepted Answer

I and III only. The source moves toward X, so wavelength is compressed (shorter) and more wavefronts arrive per second; wave speed in the medium is unchanged. So I and III only.

Question 10

When a sound source moves toward a stationary observer, the observed frequency is:

Accepted Answer

higher than the emitted frequency. Wavefronts bunch up, so observed frequency is higher.

IB Physics: Doppler effect — Practice Questions & Answers

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